The predominant current usage of the improved mechanical control cable of the present invention is as a replacement for conventional mechanical control cables in various devices including motorized machinery and motor vehicles. Mechanical cable control assemblies are a well known means of transmitting mechanical position information from a controlling device to a controlled device. Such control cable assemblies consist, in general, of an inner control cable and an outer control housing. The inner control cable has traditionally been made of either a single steel wire, or of strands of steel wire twisted together to form a cable. The outer cable housing is frequently constructed by coiling steel wire to form a tube such that the inner control cable can be slidably inserted into the outer cable housing tube. This outer cable housing is then frequently covered (sheathed) with a plastic type material.
The principle of operation of such mechanical cable control assemblies is well understood in the field. The outer cable housing is fixed at its opposite ends. The action of pulling a given amount of the inner control cable from the outer control cable housing at the controlling end results in a like amount of inner control cable being pulled into the outer cable housing at the control end. It should be noted that many mechanical control cable applications require transmittal of control position information in only one direction. Such applications include those wherein the controlled device returns to a normal position by means of a spring or other tensioning mechanism, when mechanical force is removed from the controlling end of the cable. In these applications, the outer cable housing need only be prevented from movement in one direction relative to the length of the cable assembly, and the inner control cable is only subjected to a pulling force from the controlling end. However, there are applications which require both a pushing and a pulling force from the controlling end. These latter applications, of course, require that the outer cable housing be fixed at both ends so as to prevent movement in either direction along its length. A third type of application requires that the cable transmit rotational mechanical position information. An example of this last type of application is the driving of mechanical speedometers and tachometers.
Mechanical control cable assemblies are used primarily as substitutes for rigid control rods, and the like, where either mechanical position information is required to be transmitted around turns or obstructions, or where the relative positions of the controller and the controlled device are not fixed. Therefore, it is necessary that both the outer cable housing and the inner control cable be flexible. However, It is also necessary that both the outer cable housing and the inner control cable be of a fixed length and neither easily stretchable nor compressible along the length dimension. In order to accomplish this combination of necessary qualities, the materials used in the construction of the inner control cable have been engineered so as to provide a minimum required degree of flexibility, while also resisting dimensional distortion. But, unfortunately, an ideal steel or other metallic material for this purpose can not be produced using any known technology. Repeated flexing of an inner steel control cable results in stress failure. Repeated pulling on an inner steel control cable results in stretching. Furthermore, a property of the steel that has been developed for this purpose is that it is flexible only within very restricted limits. If such steel is bent beyond these limits, it will kink or remain bent even after the bending force is removed. This results in an inner control cable that is either totally useless or severely reduced in its usefulness.
Another problem with steel inner control cables is that they require lubrication in order to slide smoothly within their outer cable housings. Such lubrication is also necessary to prevent an unacceptable wear factor between the components. However, lubricants trap dust and grit from the atmosphere which, eventually, actually contribute to increased wear. Lubricants also become stiff during cold weather and, therefore, inhibit proper operation of the device. Also, since devices which incorporate mechanical control cable assemblies are frequently exposed to the weather, moisture may be introduced inside the outer cable housing. This moisture results in the additional problems of rusting and of freezing during cold weather. This last problem has been specifically address by the invention described in U.S. Pat. No. 3,764,779 issued to Kadoya et. al, wherein heating elements are introduced into a mechanical cable control assembly to prevent freezing.
Clearly, an inner control cable which would be more resistant to stretching, which would be more resistant to kinking, which would be more resistant to stress failure under extended operation, which would not require lubrication and therefore would avoid the problems associated therewith, and which could be used to replace existing mechanical inner control cables, would be desirable. Synthetic materials have been developed which inherently have some of these properties. For instance, synthetic materials which are stronger per unit diameter than steel, and which resist stretching better than steel, are now available. Some of these same materials are also considerably more flexible than steel, and therefore inherently resist kinking and damage from excessive flexing better than steel. However, these materials have been found to be lacking in other properties necessary for their use in the construction of mechanical control cables. For instance, such materials are frequently quite susceptible to fraying and other damage caused by abrasion. Furthermore, the materials in question individually lack the resistance to compression along their length which is inherent in steel cable, and they are too prone to lateral expansion and compression to be useful in any prior art construction as mechanical control cable materials.
One material which possesses many of the qualities described above is an aramid fiber manufactured and sold by the DuPont Corporation under the name Kevlar.TM.. Kevlar fibers has been used as reinforcing material in many applications such as in automobile tires, in high pressure hoses, and in conveyor belts. Kevlar fibers have also been used as a reinforcing material in electrical cable and ropes, wherein its flexibility and resistance to stretching and corrosion have all been important qualities. However, none of these applications have required that the Kevlar.TM. fibers resist abrasion. In fact, Kevlar.TM. fibers are not inherently rugged in the respect that they will not readily withstand abrasion without damage. Further, none of these applications has required that the combined unsupported Kevlar.TM. fibers resist compression along their length. In fact, while Kevlar.TM. fibers are inherently resistant to stretching, they have little resistance to compressive forces along their length, since their flexibility will cause them to bend back upon themselves when subjected to longitudinal compressive forces. Furthermore, none of these applications has required the Kevlar.TM. fibers, or bundles of Kevlar.TM. fibers, to resist expansion or contraction across their diameter. While this is an important characteristic for mechanical control cables, it is not a characteristic of bundles of Kevlar.TM. fibers that have been created using prior technology.
Obviously, it would be desirable to incorporate the advantages of resistance to stretching and breaking, flexibility, and unit strength per diameter, which are all inherent in these synthetic fiber materials, into mechanical control cable constructions. However, prior art methods for combining these fibers have not provided the cross sectional integrity or resistance to abrasion necessary to make the fibers useful for this important purpose. No prior art construction, to the inventors' knowledge, has successfully provided a way to incorporate the advantages of modern nonmetallic materials into the construction of inner mechanical control cables. All successful mechanical control cables to date have used either a single steel wire or a twisted steel alloy wire cable as the inner control cable. Such steel inner control cables have suffered from a tendency to fail under repeated stress, a tendency to stretch, a tendency to corrode, severe limitations on flexibility, and a need for lubrication and the inherent disadvantages associated therewith.